It is known that the heart's natural pacemaker (i.e., the sinus node) for many cardiac patients, particularly heart failure patients, provides an adequate heart rate for producing a cardiac output satisfactory for low levels of exertion, but not for high levels of exertion. This is known as chronotropic incompetence. Adaptive rate pacing has, in the past, been applied to patients to improve the chronotropic response to activities.
Rate responsive pacemakers have been developed that incorporate multiple sensors for measuring physiologic demand and for developing a control signal for delivering adaptive rate pacing under appropriate circumstances. For example, rate responsive pacemakers may incorporate an accelerometer for detecting patient movement and a minute ventilation sensor for detecting respiratory activity as an indicator of physiologic demand. When a patient having such a pacemaker implanted goes from a resting state to a minimal level of activity, the accelerometer will respond to the patient's stepping rate and the minute ventilation sensor will respond to the increased metabolic demand resulting in an increased paced heart rate from one or both of the sensors.
The physiologic sensors of rate responsive pacemakers provide inputs to a microprocessor-based controller for adjusting the rate at which cardiac pacing pulses are delivered to meet physiologic demand. It is difficult to predict an appropriate pacing function capable of generating a paced rate corresponding to a patient's metabolic demand at the time of implanting the pacemaker in the patient. Notwithstanding the efficacy of conventional rate responsive pacemakers, adjustments to the pacing rate by the microprocessor-based controller are made using a response factor that is based on estimated activity levels and target heart rates. Inappropriately computing or adjusting the response factor may result in a paced rate that is too high or too low for a given metabolic demand. If the paced rate is too high, the patient may feel palpitated or stressed. If too low, the patient may feel fatigued, tired or dizzy.